ATS 621 Fall 2012 Lecture 13

Methane

Fate of peroxy radicals (HO2., RO2.)

The importance of NOx

Where are the net ozone production and destruction occurring Where are the net ozone production and destruction occurring? (remote troposphere) Model simulations of present day NOx and O3 distributions (Jacob, chapter 11) From Jacob text: Ozone concentrations generally increase with altitude, mainly because of the lack of chemical loss in the upper troposphere (water vapor and hence HOx concentrations are low). Higher O3 concentrations are found in the northern than in the southern hemisphere, reflecting the abundance of NOx.

Oxidizing capacity of the troposphere The anthropogenic influence on OH is complicated. Increasing NOx and O3 act to increase OH, while increasing CO and hydrocarbons act to deplete OH (section 11.3). Because CO and CH4 have longer lifetimes than NOx and O3, their anthropogenic enhancements are more evenly distributed in the troposphere. It is thus found in the model that the net effect of human activity is to increase OH in most of the lower troposphere and to decrease OH in the upper troposphere and in the remote southern hemisphere (Figure 11-6). There is compensation on the global scale so that the global mean OH concentration as defined by (11.5) decreases by only 7% since preindustrial times (other models find decreases in the range 5-20%). The relative constancy of OH since preindustrial times is remarkable in view of the several-fold increases of NOx, CO, and CH4. There remain large uncertainties in these model analyses. From the CH3CCl3 observational record, which started in 1978, we do know that there has been no significant global change in OH concentrations for the past 20 years. [Jacob text]

Does NOx in remote ocean regions represent “clean, unperturbed background”? Jacob text: NO, “Recycling of NOx through PAN maintains NOx concentrations in the range of 10-50 pptv throughout the remote troposphere.” PEROXYACETYLNITRATE (PAN)

DECOMPOSITION OF PAN LEADING TO OZONE PRODUCTION Chemistry: Changing NOy speciation Origin: Warm Conveyor Belt over Asia ECMWF Evolution: Split by blocking high pressure O3 H CO Slide courtesy C. Heald [Heald et al., 2004]

Formation of PAN In smog: PAN is eye, lung irritant; damaging to plants

Alkane reaction mechanisms

Alkane reaction mechanisms, continued

Alkoxy radicals Notice formation of carbonyls, which can photolyze

Alkene mechanisms

Aromatic mechanisms Glyoxal & methylglyoxal common in aged, urban plumes

Building atmospheric chemical mechanisms

EXTRAS (slides courtesy Prof. Colette Heald)

http://tes.jpl.nasa.gov/mission/ozone/

CARBON MONOXIDE IN ATMOSPHERE Source: incomplete combustion Sink: oxidation by OH (lifetime of 2 months)

SATELLITE OBSERVATION OF CARBON MONOXIDE MOPITT CO (2000)

SATELLITE OBSERVATIONS OF BIOMASS FIRES (1997)

GLOBAL DISTRIBUTION OF CO NOAA/GMD surface air measurements

SPACE-BASED METHANE COLUMN OBSERVATIONS by solar backscatter at 2360-2385 nm

GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements Sink: oxidation by OH (lifetime of 10 years)

HISTORICAL TRENDS IN METHANE The last 30 years The last 1000 years

NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE Zeldovich Mechanism: combustion and lightning At high T (~2000K) oxygen thermolyzes: O2  O + O O + N2  NO + N N + O2  NO + O FOSSIL FUEL 23.1 AIRCRAFT 0.5 BIOFUEL 2.2 BIOMASS BURNING 5.2 SOILS 5.1 LIGHTNING 5.8 STRATOSPHERE 0.2

LIGHTNING FLASHES SEEN FROM SPACE (2000) DJF JJA

USING SATELLITE OBSERVATIONS OF NO2 TO MONITOR NOx EMISSIONS SCIAMACHY data. May-Oct 2004 (R.V. Martin, Dalhousie U.) detection limit

PAN NO3 N2O5 NO2 NO HNO3 NOX CYCLING hn Combustion lightning ~ 1 day Example of PAN formation from acetaldehyde: CH3CHO + OH  CH3CO + H2O CH3CO + O2 + M  CH3C(O)OO + M CH3C(O)OO+NO2 + M  CH3C(O)OONO2 + M PAN carbonyl oxidation T NO3 O3 O3 M O2 hn N2O5 NO2 NO Combustion lightning H2O OH, M HO2 HNO3 O3 ~ 1 day

GLOBAL BUDGET OF TROPOSPHERIC OZONE Chem prod in troposphere, Tg y-1 4300 1600 Chem loss in troposphere, 4000 Transport from stratosphere, 400 Deposition, 700 Burden, Tg 360 230 Lifetime, days 28 42 Present-day Preindustrial O2 hn O3 STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, VOC NO+peroxy radicals is rate-limiting, so:

GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE Climatology of observed ozone at 400 hPa in July from ozonesondes and MOZAIC aircraft (circles) and corresponding GEOS-Chem model results for 1997 (contours). GEOS-Chem tropospheric ozone columns for July 1997. [Li et al., 2001]

1996-2005 NOx EMISSION TREND SEEN FROM SPACE [Van der A et al., 2008]

POWER PLANT EMISSION REDUCTIONS IN THE EASTERN US Effects of NOx controls on large point sources in the Eastern US beginning in the late 1990s Acid Rain Program, NOx SIP Call, NOx Budget Trading Program Focus on coal-burning power plants Improved burner technology, post-burner ammonia scrubbers Northeast Urban Corridor E(NOx) < 20% power plant Ohio River Valley 1997 E(NOx) ~ 50% power plant Ohio River Valley 2005 E(NOx) ~ 20% power plant Courtesy: Greg Frost (NOAA) [Kim et al., 2006]

POWER PLANT POINT SOURCES IN WESTERN US SEEN FROM SPACE North Valmy Intermountain Hunter / Huntington Mohave Navajo Four Corners/ San Juan Cholla/Coronado/ Springerville Bonanza Craig/Hayden Jim Bridger/ Naughton Dave Johnston/ Laramie River Colstrip Reid Gardener Courtesy: Greg Frost (NOAA) [Kim et al., 2009]